Convective system

ABSTRACT

We have discovered a simple electrical assembly of heating coils which may be arranged to produce hot air or gas up to 1500° C. This assembly is the invention claimed in this patent.

[0001] Heating can be carried out by conduction, radiation or convection. A wide variety of thermal processing applications are found throughout industry. Heat treating, joining, curing and drying operations are carried out in many different types of furnaces and ovens. The method of heating is normally a radiative technique with radiant electric heating elements placed along the walls of the furnace. Although such a method is efficient for temperatures above 1000° C. (1832° F.), the use of convection as the heat transfer mechanism is normally thought to be more efficient in the lower temperature ranges. One of the best devices for Air heating is an Airtorch™. The following patents all pertain to best methods of heating air namely U.S. Pat. Nos. 5,766,458, 5,655,212, 5,963,709. Discussions on convective heating are available from (1) M. Fu, Kandy Staples and Vijay Sarvepalli. A High Capacity Melt Furnace for Reduced Energy Consumption and Enhanced Performance. Journal of Metals (JOM), May 1998, pg 42 and (2) ADVANCE MATERIALS & PROCESSES magazine (pages 213 to 215, October, 1999).

[0002] The proper selection of thermal processing ovens and furnaces is a critical decision in order to meet the needs almost all engineering products during their manufacture. Oven and furnace design must take into consideration heat transfer methods, the temperature uniformity, movement of the product, atmosphere, construction and the heat generation method. Heat processing equipment is usually classified as ovens operating to 1000° C. and as furnaces above this temperature. Batch and continuous designs are the common choices depending on the flexibility and productivity requirements. The source of heat is normally provided by oil, gas or electricity. The batch oven is the largest type of design used to manufacture the product. The continuous oven operates on a continuous or pusher basis. There are similarities between the batch and continuous, each having characteristics depending upon product design and processing conditions.

[0003] Heat transfer to the work can be natural convection, forced convection or by radiation. Natural convection is slow and not very uniform. Forced convection on the other hand is easily controllable and can be directed for odd shapes. Radiant heat transfer at higher temperatures may be faster for some products but may contribute other problems to the products like non-uniformity and distortion, to mention a few. Forced convection offers advantages over radiant heating for a number of manufactured products. It is well known in the art that convective heating eliminates these problems

DISCOVERY

[0004] We have discovered a new technique for very low cost convective heat generation. The basic method is to heat the air or gas through a concentric energized heating coil system as shown below in the diagram (FIG. 1). The concentric design is important as without it the air is not heated to the same temperature.

[0005] The following tests were done with (1) metallic wire and (2) with molybdenum disilicide wire and the following results were obtained.

[0006] 1. Metallic Wire

[0007] Coil material commonly available metallic heating wire made of Nickel Chromium alloy or Fe—Al—Cr or Fe—Al, Ni—Cr alloy. Generally metallic wires can be heated in air to about 1200° C. Wire diameters from 0.1 mm to a 1.2 mm were tried for the experiments. We conducted the following experiments with the Fe—Al—Cr alloy (alloys made of Fe—Al—Cr—Nb or Fe—Al—Cr—Mo—Nb are expected to perform similarly as are other metallic & intermetallic systems):

[0008] The best stable experiment to date where the air was heated to 850° C. at a 3.5 scfm flow rate had the following design features. Other experiments were also conducted where air was heated to close to 1000° C., however very long term tests at these higher experiments have not yet been carried out.

[0009] Wire diameter 1.2 mm

[0010] Outer wire separation (pitch) 0.285 mm

[0011] Inner wire separation (pitch) 0.285 mm

[0012] Winding: Opposite direction in inner and outer coil

[0013] Gap between coils: 5.23 mm

[0014] The thermocouple was located at about 3 mm from the exit nozzle plane. When located at the exit nozzle plane the thermocouple read even up to 980° C. It is expected that the upper range with metallic elements will be about 1000° C. for air temperature. Other gases depending on their thermal properties will have a different exit temperature. If the metallic elements are made of Mo, W or other such higher temperature metals the exit temperature could be much higher.

[0015] Based on these results the device in FIG. 2 was constructed as a prototype.

[0016] We contemplate that the wire sizes for the inner and outer coils could be different for different products. Similarly the pitch can be different for each coil and different at different locations in the same coil.

[0017] Where the contacts are made to the incoming power supply line it is contemplated that the pitch would be larger than at the main heating sections of the coils in order to keep the contacts cool. Spacers and other inserts between the coils are contemplated if required.

[0018] It is thought that the presence of the inner coil serves to overcome the “conda” effect and thus improves contact with the air.

[0019] Some further experiments were conducted:

[0020] Coil design was adjusted with the appropriate physics in mind.

[0021] Experiment 1: Outer coil provides rifling which increases heat transfer from the elements to air.

[0022] A helical coil of 240 mm long×13.2 mean dia, working out for 8.2 Ohms (18SWG A1 commercial wire) was used for testing. The coil was inserted in open-ended ceramic tube. The exit end of the coil was brought back to the inlet side through a ceramic insulating tube. (Refer FIG. 2). The coil was operated at 110 V, at a power rating of 1.47 kW. The airflow was maintained at 5 SCFM @ 0.4 Kgs/cm² working pressure. The exit temperature of the air stabilized at 560° C.

[0023] Experiment 2: Inner coil over comes conda effect, and provides for annular area heating of air, which provides for the highest heat transfer to the air.

[0024] Furthering on experiment 1, the exit end of the coil, was wound on its return on the ceramic insulating tube (Refer FIG. 2). The resulting coil resistance was 10.8 Ohms. The coil was operated with the same airflow, air pressure and operating voltage of 110 V. The coil now operated at 1.1 kW, and the exit temperature stabilized at 806° C.

[0025] Experiment 3: Inner coil winding in the opposite direction of the outer coil gives opposite rifling with that of the outer coil. This causes a turbulence effect on the airflow, which increases heat transfer to the air. Furthering on experiment 2, the inner coil was wound in the opposite direction of the outer coil. All parameters were the same as Experiment 2. The exit temperature stabilized at 845° C.

[0026] Note that the opposite winding configuration gave a nearly 50° C. higher temperature. Table 1 below gives further experimental details and exit temperatures. TABLE 1 Airflow cross Exit Experiment Coil section Air temperature Number resistance Voltage Current area Power Air Flow Pressure of air Experiment 1 8.2 110 13.4 25.1 mm2 1.47 kw   5 SCFM   7 Kg/cm2 560 Experiment 2 10.8 110 10 17.2 mm2  1.1 kw   5 SCFM   7 Kg/cm2 806 Experiment 3 10.8 110 10 17.2 mm2  1.1 kw   5 SCFM   7 Kg/cm2 845 Experiment 4 11.0 110 10 55.2 mm2  1.1 kw 3.5 SCFM 0.4 Kg/cm2 850

[0027] 1. Molybdenum Disilicide Alloy Wire

[0028] Molybdenum disilicide wires can be heated in air to 1900° C. but are more brittle than metallic wire. The coils were obtained from Micropyretics Heaters International Inc. who are the leading experts for molybdenum disilicide in the US.

[0029] Wire diameter 3 mm, 4 mm or 5 mm may be used. An experiment was conducted with outer wire separation (pitch) 12.7 mm and inner wire separation (pitch) 12.7 mm.

[0030] Gap between coils tested was varied from 4 mm to 15 mm. Best results were obtained with the 5 mm wire.

[0031] Best results gave a temperature of 1165° C. to 1400° C. at different measurement positions with 1400° C. as set point on the controller and airflow set to 1 scfm. (Table 2). Note this configuration is trademarked Ultratorch™.

[0032] Best results show a temperature of 1332° C. to 1500° C. at different measurement positions with 1500° C. as set point on the controller and airflow set to 1 scfm. (Table 3). Note this configuration is trademarked Ultratorch™.

[0033] These hitherto-fore unavailable very high temperatures in gasses for transferring to parts has only been possible because of the new coil in coil design with the proper spacing and gaps provided that the two coils are electrically connected. It is also found that opposite winding in the inner and outer coils give rise to very high temperatures of the gas at the exit. (In the claim below cfm refers to cubic feet per minute)

[0034] The typical uses of such a device is in low cost heating. Three different types of use classes are considered.

[0035] 1. Heating of a chamber such as an oven or furnace which may or may not have other heating systems in it.

[0036] 2. Heating of a fluid passing though the coils

[0037] 3. Heat directed at a surface to cause heating of surface for applications such as coatings, hardening, debinding, glowing, etc.

[0038] In the claims below the coils are defined as those which can be electrically heated or heated by a combination of electric and other thermal methods. The coils can be metallic, molybdenum disilicide, silicon carbide, intermetallic, or ceramic. 

We claim:
 1. A novel gas heater product, with a coil in coil heater assembly, and annular space for the gas where the inner and outer coils are connected electrically and the gas flow rate varies from 1 cfm to about 1000 cfm.
 2. A novel gas heater product, with a coil in coil heater assembly, and annular space for the gas where the inner and outer coils are connected electrically and the wires in the inner and outer coil are wound in an opposing manner and the gas flow rate varies from 1 cfm to about 100 cfm.
 3. The product of claim 1 or claim 2 where the gap between the coils is ranges from about 5 mm to about 14 mm.
 4. The product of claim 1 or claim 2 wherein the ratio of the gap to the winding spacing ranges from about 4.5 mm to about 0.5 mm.
 5. The product of claim 1 wherein the coils are wound in the same direction.
 6. A product of claim 1 where the inner and outer diameter coil wires may be varied from about 0.1 mm to about 6 mm
 7. A product of claim 2 where the inner and outer diameter coil wires may be varied from about 0.1 mm to about 6 mm
 8. The product of claim 1 where the cross sectional area for air flow is about 15 to about 150 square mm
 9. The product of claim 2 where the cross sectional area for air flow is about 15 to about 150 square mm.
 10. The product of claim 1 where multiple coil and configurations are used in one housing.
 11. The product of claim 2 where multiple coil and configurations are used in one housing. TABLE 2 Typical Results of the UltraAirtorch ™ UAT5 Ref: p83 (4) HIPAN Primary: 208 Volts, Secondary: 40 Volts tap. Jun. 28, 2002 Temperature, C. Flow, Secondary Primary Time Set point Process SCFM Current Volts Current Volts Comments 10:00   0 RT 2.0 0 0 0 0 Started 10:03 1400  542 2.0 93 14 16 10:05 1400 1167 2.0 103 21 18 10:07 1400 1371 2.0 95 21 18 10:08 1400 1400 2.0 106 18 15 10:20 1400 1402 2.0 105 18 18 10:30 1400 1400 2.0 79 16 14 10:38 1400 1400 2.0 77 16 13 10:38:50 1400 1400 3.0 86 18 14 10:48 1400 1400 3.0 86 17 14 10:58 1400 1400 3.0 81 16 14 11:08 1400 1400 3.0 81 16 15 11:08:50 1400 1400 4.0 89 18 16 81 11:20 1400 1400 4.0 96 19 17 End

TABLE 3 Typical Results of the UltraAirtorch ™ UAT5 Ref: p95 (4) HIPAN Primary: 240 Volts, Secondary: 40 Volts tap. Jul. 30, 2002 Temperature, C. Flow, Secondary Primary Time Set point Process In-situ SCFM Current Volts Current Volts Comments 9:35   0 RT RT 3.0 0 0 0 0 Started 9:39 1050 1046 621 3.0 89 13 15 9:42 1372 1334 942 3.0 102 19.6 18 9:43 1372 1372 1032 3.0 95 18.5 17 9:47 1372 1372 1055 3.0 123 22 19 End 10:47 1400  392 432 3.0 0 0 0 0 Restarted 10:49 1400 1042 702 3.0 124 19.7 22 10:50 1400 1375 954 3.0 98 18.8 17 10:51 1400 1397 1022 3.0 95 16 10:52 1400 1400 1074 3.0 89 17 16 11:00 1400 1400 1165 3.0 81 15 11:10 1500 1500 1279 1.0 70 12 11:13 1500 1500 1301 1.0 67 14 12 81 11:18 1500 1500 1314 1.0 66 12 12 11:26 1500 1500 1316 0.5 56 11 10 11:28 1500 1500 1315 1.0 60 12 10 11:39 1500 1500 1316 1.0 58 11 10 88 11:53 1500 1500 1322 1.0 57 11 10 69 12:05 1500 1500 1322 1.0 56 11 10 69 12:55 1500 1500 1324 1.0 55 11 10 1:31 1500 1500 1324 1.0 55 11 10 2:05 1500 1500 1328 1.0 55 11 10 3:30 1500 1500 1332 1.0 55 11 10 5:00 1500 1500 1332 1.0 55 11 10 70 End 